The present invention relates to a method of making semi-finished products and other objects using a material which has been strengthened by means of dispersion hardening.
Metallurgy and the physics of metals have taught the development of methods for controlling the properties of metal to be used as raw material for specific purposes. However, it is common knowledge that a change in one property affects other properties of the material, and an improvement of one is quite often accompanied by a deterioration of others. Thus, the balance of properties is often a compromise.
Take, for example, copper as a typical historical example. It has good thermal and electrical properties, but it is mechanically poor. Copper has been strengthened (already in ancient times), but the electrical and thermal properties deteriorate e.g. upon alloying. Of course, that depends to some extent on the method used for strengthening.
Metals have been strengthened e.g. hardened in various ways, such as by deformation of the structure during cold working, (strain hardening); strengthening by mixed crystal; precipitation hardening; strengthening by fiber reinforcement; and strengthening by means of dispersion, i.e. by causing dispersion and inclusion of particularly hard particles in the metal.
An essential function of either method is to impede any movement of dislocations in the crystal structure. Strengthening and hardening by dispersion operates on the principle that embedded particles (impurities) impede the migration and movement of dislocations in the crystal structure. These particles prevent such migration during deforming as well as during annealing at temperature above the recrystallization temperature. The impediment is so strong that even at temperatures up to 90% of the absolute melting point neither recrystallization nor a reduction of the tensile strength can be observed. Upon deforming (working) dislocations are stopped by the embedded particles and can pass only when the acting forces (stress) is greatly increased. The migration of grain boundaries is prevented in the same manner, even at very high temperatures so that growth of grains is, in fact, suppressed.
Strengthening of metal by means of dispersion hardening is considerably better than cold working or the mixed crystal method, because the strength is actually improved to a greater extent and the electrical properties e.g. of copper deteriorate to a lesser degree.
A disadvantage of the dispersion method is that e.g. oxygen requires a long time to diffuse into the solid copper. By way of example, sufficiently dense and homogenic internal oxidation in a copper-aluminum alloy requires periods for diffusion of the oxygen in the order of 100 hours per cm wall thickness and at temperatures of about 1000.degree.C.
A paper published in "Metall", Vol. 24, of May 1970, issue No. 5, page 465, et seq. proposes dispersion hardening of thinner metal parts and compacting them subsequently.